The pathway of peptidoglycan biosynthesis is both essential and unique to bacteria, making it an attractive target for antibiotic research. Several enzymes in this pathway are molecular targets of naturally occurring antibiotics such as fosfomycin, cycloserine, b-lactams and vancomycin.
The construction of the peptidoglycan begins in the cytoplasm with an activated sugar molecule, UDP-N-acetylglucosamine. After two reactions (catalyzed by MurA and MurB) that result in the placement of a lactyl group on the 3—OH of the glucosamine moiety, a series of ATP-dependent amino acid ligases (MurC, -D, -E, and —F) catalyze the stepwise synthesis of the pentapeptide sidechain using the newly synthesized lactyl carboxylate as the first acceptor site. After attachment of the sugar pentapeptide to a lipid carrier in the plasma membrane, another glucosamine unit is added to the 4—OH of the muramic acid moiety. The completed monomeric building block is moved across the membrane into the periplasm where the penicillin-binding proteins enzymatically add it into the growing cell wall (Lugtenberg, E. J., 1972, Studies on Escherichia coli enzymes involved in the synthesis of Uridine Diphosphate-N-Acetyl-Muramyl-pentapeptide. J. Bacteriol. 110:2634; Mengin-Lecreulx, D., B. Flouret, and J. van Heijenoort, 1982, Cytoplasmic steps of peptidoglycan synthesis in Escherichia coli. J. Bacteriol. 151: 1109-1117).
Because the pentapeptide sidechain is not synthesized ribosomally it contains more diverse chemical functionality than a typical peptide, both structurally and stereochemically. Two of the enzymes catalyze the addition of D-amino acids (MurD and MurF) and MurE mediates the formation of a peptide bond between the g-carboxylate of D-glutamate and the amino group of L-lysine. Presumably these structures render the exposed peptidoglycan resistant to the action of proteases, but they also imply that the active sites of the enzymes must have unusual structures in order to handle the somewhat uncommon substrates. These unusual active sites are targets to bind novel inhibitors that can have antimicrobial activity.
Although peptidoglycan assembly is a proven target for antibiotics, there are no known inhibitors for many of the enzymes of the pathway. Since these enzymes are conserved among eubacteria, inhibitors of this pathway are likely to be broad spectrum antibiotics. Among these potential enzyme targets is MurF, UDP-N-acetylmuramyl-L-alanine-D-Glutamine-m-Dap: D-alanine-D-alanine ligase. This enzyme is a target for the antibiotic cycloserine (Kleinkauf H and H. von Dohren. 1990. Nonribosomal biosynthesis of peptide antibiotics. Eur J. Biochem. 192:1-15). This validates the assumption that inhibitors of this enzyme are likely to lead to antibiotics for treating infections with either Gram (−ve) or Gram (+ve) bacteria.